Does calcium ameliorate the negative effect of NaCl on melon root water transport by regulating aquaporin activity?

The hydraulic conductance (L₀ ) of detached, exuding root systems from melon (Cucumis melo cv. Amarillo oro) was measured. All plants received a half-strength Hoagland nutrient solution, and plants stressed either solely with NaCl (50 mM) or with NaCl (50 mM) following treatment (2 d) with CaCl₂ (10 mM) were compared with controls and CaCl₂ -treated (10 mM) plants. The L₀ of NaCl-treated plants was markedly decreased when compared to control and CaCl₂ -treated plants, but the decrease was smaller when NaCl was added to plants previously treated with CaCl₂ . A similar effect was observed when the flux of Ca²⁺ into the xylem and the Ca²⁺ concentration in the plasma membrane of the root cells were determined. In control, CaCl₂ - and NaCl + CaCl₂ -treated plants, HgCl₂ treatment (50 μM) caused a sharp decline in L₀ to values similar to those of NaCl-stressed roots, but L₀ was restored by treatment with 5 mM DTT. However, in NaCl roots only a slight effect of Hg²⁺ and DTT were observed. The effect of all treatments on L₀ was similar to that on osmotic water permeability (P_f ) of individual protoplasts isolated from roots. The results suggest that NaCl decreased the passage of water through the membrane and roots by reducing the activity of Hg-sensitive water channels. The ameliorative effect of Ca²⁺ on NaCl stress could be related to water-channel function.